Cardiac Arrest is defined as a heartbeat, which does not result in efficient pumping of blood. In many cases, lack of carotid pulse is the gold standard for diagnosing cardiac arrest, but pulselessness (particularly in the peripheral pulses) may be a result of other conditions.
In a hospital or ambulance, cardiac arrest is identified by the lack of a pulse (or lack of heartbeat if listened to through a stethoscope), and advanced life support is given.
Out of hospital, lay rescuers identify cardiac arrest in a number of ways. In the USA, most lay rescuers are trained to check for a carotid pulse, whereas in the UK, rescuers are taught that a lack of normal breathing is evidence of cardiac arrest, and they begin CPR without checking a pulse.